1. Field of the Invention
The present invention generally relates to fuses used in semiconductor technology for implementation of redundancy or custom wiring and, more particularly, to fuses exhibiting reduced blow-current requirements.
2. Description of the Prior Art
Redundancy in integrated circuit memories is part of current wafer and chip manufacturing strategy to improve yield. By providing redundant circuits on chips, integrated circuit memory yields are increased by eliminating from circuit operation those circuits or modules which are defective or are not needed. The practice is to blow fuses which allow redundant memory cells to be used in place of cells that are nonfunctional. In the manufacture of integrated circuits, it is also common practice to provide for customization of chips and modules to adapt chips to specific applications. In this way, a single integrated circuit design may be economically manufactured and adapted to a variety of custom uses.
Fuses have been used in lower performance products where the method of blowing the fuses is with a laser. This is not practical in high performance products and, therefore, the preferred method of blowing the fuses is by means of high currents. Typically, fuses or fusible links are incorporated in the integrated circuit design, and these fuses or fusible links are selectively blown by passing an electrical current of sufficient magnitude through them to cause them to open. For example, U.S. Pat. No. 3,959,047 to Alberts et at. discloses a metalized fuse construction in the form of straight links which are "necked" to cause a high current concentration to heat and open the links. An on-chip programmable polysilicon fuse is described in IBM Technical Disclosure Bulletin, vol. 29, no. 3, Aug. 1986, pp. 1291, 1292, and a tungsten/aluminum fuse blown by electromigration is described in IBM Technical Disclosure Bulletin, vol. 31, no. 5, Oct. 1988, pp. 347, 348.
The technology for high density, very large scale integrated (VLSI) circuits is being pushed to dimensional limits. In the future it may be extremely difficult to form fuses in a manner in which "necking" or constrictions can be used to force a localized current to open a fuse. Moreover, the "real estate" of the chip itself, always a valuable commodity, is becoming even more "expensive" with ever increasing transistor densities. This means that the power supplies and driver transistors required to blow the fuses must compete for space on the chip with functional circuitry of the chip. For example, using deep submicron design, P+ polysilicon straight fuses require approximately 10 mA/fuse to be blown-open. As a result, the fuse driver transistors must have a width of at least 30 .mu.m and require a supply voltage of 5 V. In one application, the straight current-blown fuses and their drivers would add approximately 1.9% to the chip area, and the high voltage required for fuse blow makes programmability in a system environment costly.
The relatively high current required for fuses having straight geometry to be blown-open constrains the minimum size of the fuse drive transistors and/or their supply voltage. Module level programmability requires a supply voltage which is higher than the normal operating voltage and very large driver transistors. If in-system programmability is desired, providing the higher than normal operating voltage for blowing the straight fuses is costly.
U.S. Pat. No. 4,064,493 to Davis discloses a low-current fusible programming link which depends on electromigration to blow the link. Location of the link in proximity to a collector-base junction aids in heating the link, thereby reducing the current required to open the link. Davis' preferred fuse material is aluminum for enhanced electromigration effect. The design of the fuse link therefore is limited to both material and proximity to a semiconductor junction to provide heat. This considerably restricts the circuit design and limits the applications of the Davis fuse link.